One of the exciting developments in the area of high technology in the recent past concerns hydrophobically modified polymers (HMPs) which are able to self associate in aqueous media and form reversible networks with spectacular viscoelastic properties such as enhanced viscosification efficiency, shear thickening property, shear and salt stability. Therefore, they find extensive applications in enhanced oil recovery, cosmetic lotions, paints, coatings, food additives and pharmaceuticals. These polymers consist of a hydrophilic backbone with a small number of hydrophobic groups dispersed along the chain or terminally situated on the chain. There are two ways to incorporate the hydrophobic moities in the water-soluble polymer i.e. the direct copolymerization of hydrophobic monomers with water-soluble monomers or the post modification method using water-soluble polymers. (Water-Soluble Polymers: Synthesis, Solution Properties and Applications, Edts. S. W. Shalaby, C. L. McCormick, G. B. Butler, ACS Symp Ser. 467, Washington, 1991; Polymers in Aqueous Media: Performance Through associations Edt. J. E. Glass, Adv Chem ser. 223, ACS, Washington, 1989; Macromolecular Complexes in Chemistry and Biology Edts. P. Dubin, R. M. Davies, D. N. Schultz, C. Thies, Springer Verlag, Berlin 1994; L. M. Landoll, U.S. Pat. No. 4,228,277, 1980)
Various molecular architectures such as random, graft, block and hydrophobically end-capped polymers have been envisaged in designing hydrophobically modified polymers.(Iliopoulos et.al., Langmuir 7, 617, 1991; J. E. Glass et.al. Macromolecules, 26, 5149, 1993; Ma and Cooper, Macromolecules, 34, 3294, 2001, Arotcarena et.al., JACS, 124, 3787, 2002, McCormick et.al., Polymer, 29, 731, 1988; Hill et.al., Macromolecules, 26, 4521, 1993)
Although, there are fast expanding academic studies on associating polymers, their industrial development has not really taken place and only a small number of polymers are commercially available. Therefore, there is a need to develop newer HMPs for specific end applications.
Hydrophobic association plays a dominant role in governing co-operative aggregation process in aqueous environment. For example, surfactant self assembly, protein folding, formation of biological membranes and molecular recognition.
The hydrophobic compounds constitute alkyl substituted phenols and cyclohexanols. The alkyl substituted phenols can be derived from renewable resources such as, CNSL which is available abundantly in India and is inexpensive.
Hydrophobically end-capped polymers contain hydrophobic moieties having 8-18 C-atoms (alkyl, perfluoroalkyl) or aromatic groups. The main representatives are the Hydrophobically modified Ethoxylated URethanes (HEUR), which are obtained by coupling poly(ethylene glycol) and alkylamines with diisocyanates. These are one of the earlier HMPs prepared and have undergone major industrial development. HEURs have been widely studied from a fundamental point of view as model associative thickeners. HEUR polymers offer the greatest variation in architectural design of any hydrophobically modified polymers. In the case of graft copolymers, the hydrophobic groups such as alkyl, perfluoroalkyl or aromatic rings are distributed along the hydrophilic backbone chains. For example, the modifications of polysaccharides is generally performed through the functionalization of hydroxyl groups with alkyl substituents. Number of such systems like HM-pullulan (Akiyoshi, et.al Chemistry Lett, 1727, 1992), guar (Aubry and Moan, J. Rheol, 38, 1681, 1994), alginates (Sinquin et.al., Langmuir, 9, 3394, 1993), and cellulose have been reported in the literature. In the case of synthetic polymers, polymers based on polyacrylamide, polyacrylic acid, polymethacrylic acid have been reported. Some graft copolymers namely, Hydrophobically modified Alkali Swellable/Soluble Emulsion were developed at Union Carbide Corporation, USA and were prepared by emulsion copolymerization technique using methacrylic acid, ethyl acrylate and a hydrophobic comonomer. Tirtatmadja, et.al., (Macromolecules, 30, 3271, 1994) and Wang et. al., (Polym. Bull. 20, 577 1988) have reported the associating polymers based on the grafting of alkyl amines on the PAA backbone chain using Dicyclohexyl carbodiimide (DCCI), coupling agent in an aprotic solvent. The solution properties of these polymers were found to depend on the degree of hydrophobic content and the alkyl chain length. The influence of surfactants and salts on the viscometric behaviour of the modified PAA solutions were also reported. Other HMPs based on modified PAA were also reported. During the same time, the concept of thermoassociating polymers was evolved. This concept, based on the switch properties of macromolecular side chains, which is characterized by a lower critical solution temperature (LCST), was generalized with a large set of copolymers designed with thermosensitive side chains such as PEO, PPO, PNIPAM grafted onto water soluble backbones such as PAA, PAM and other polyelectrolytes (Maroy P., Hourdet D., L'Alloret and Audebert R., Eur. Patent 0 583 814 A1 1993; Durand A., Herve M., Hourdet D., in “Stimuli-Responsive Water Soluble and Amphiphilic Polymers” C. L. McCormick Ed., ACS Symposium Series 780, Chapter 11, 181-207, 2000). These polymers were studied in aqueous solutions with various added co-solutes such as salts, neutral species and anionic surfactants. In aqueous solution these polymers provide an enhancement of the viscosity on heating, the extent of which can be controlled by polymer concentration, grafting ratios, salt concentrations etc. Along the same lines, polyaspartic acid modification has also been reported (Polym. Bull. 44, 385, 2000, Polym. Bull. 45, 39, 2000). An excellent review on the hydrophobic association in perfluorocarbon containing water-soluble polymers has been reported. (Hogen-Esch and Amis, Trends in Polym Sci, 3, 98, 1995). Synthesis of fluorocarbon modified poly(acrylic acid) in supercritical CO2 has been reported (Polymer, 43, 6357, 2002). In aqueous solution and above a certain threshold polymer concentrations, the hydrophobic groups form interchain aggregates that stabilize a transient network. As a result, the viscosity enhancement is observed. Addition of surfactants and salts to the aqueous solutions of HMPs further enhance their thickening properties. (Iliopoulos et.al., Langmuir, 617, 1991; Zhuang et.al., 43, 2075, 2002; Magny et.al., Polymer, 33, 3151, 1992; Morishima, Prog. Polym. Sci., 15, 949, 1990; Ringsdorf et.al., Macromolecules, 25, 7306, 1992).